Conventional silicon-based temperature sensors use the relationship between current, voltage, and temperature to detect or infer ambient temperature. Silicon-based temperature sensors include temperature sensors based on a base-emitter voltage (VBE) or change in base-emitter voltage (delta VBE) across a bipolar transistor, a voltage across or current flowing through a diode, a change in resistance of a resistor, or the change in a voltage or current across a MOSFET transistor.
Process variation, mismatch, and package-induced stress result in offset and/or scale factor errors in these conventional sensors. Furthermore, many applications require a digital temperature measurement and the temperature sensor output is often an analog quantity. The analog temperature must therefore be converted into a digital signal via an analog-to-digital converter (ADC) in conjunction with a voltage reference. Variations in the scale factor and offset of the ADC as well as in the voltage reference cause errors in the digital output signal.
Calibration of silicon-based temperature sensors at a time of manufacture may be used to improve the accuracy of the temperature sensors. However, it is difficult to apply controlled, accurate temperatures to the device under test (DUT) because thermal systems are intrinsically distributed systems that are hard to isolate from the surrounding environment. Thus, conventional temperature sensors are limited in their achievable accuracy.
Wafer-probe systems are relatively well-suited for applying a temperature to the DUT due to the comparatively large and uniform thermal mass of the chuck (which is isolated from the environment on all sides by air) and the thin layer of silicon. However, calibration before packaging is of only limited use because the packaging process can cause a shift in the temperature characteristics of the DUT. Attaining an accurate temperature calibration at final test following packaging is a quixotic effort due to thermal diffusion paths through the work press, socket pins to the loadboard, and loadboard to the tester head and environment. In practice it is difficult to maintain absolute temperature accuracy of one degree Celsius or less in a manufacturing environment at room temperature. Accurate application of a temperature, whether hot or cold (e.g. for multi-point temperature calibration), is even harder due to the thermal gradients caused by conduction paths to ambient.
Furthermore, in applications where heat is generated on the same piece of silicon as the temperature sensor, a further source of error exists due to thermal resistivity between the location where the heat is generated, through the package and socket, and the location of the heat sink (e.g. tester head).